The present invention relates to a system and apparatus for removing particulate matter from liquid suspension, and more particularly to an improved method and apparatus for recovering tall oil soap particles from black liquor produced from wood pulping processes.
A by-product of the wood pulping industry is a black, viscous substance known as "black liquor" which is obtained from pulp digested via pulp washers and/or evaporators. This black liquor is typically burned in order to recover its soda value which is then in turn reused in the wood pulping process. Suspended in this black liquor are tall oil soap particles which are useful in their own right in other industries. Consequently, it has been the practice to recover as much of the tall oil soap particles as possible before burning the black liquor.
Tall oil soap is recovered from the black liquor by feeding the black liquor into a skimming tank, where the tall oil soap particles float to the surface of the liquor and may be skimmed off. Since, for efficiency of operation, the black liquor cannot be held in the skimming tanks for too great a period of time, some of the tall oil soap particles which are suspended in the black liquor never reach the surface of the liquor before being drained from the tank. Although this residue is fairly small, when viewed on a cumulative basis it represented a significant loss in tall oil soap.
To reduce the amount of tall oil soap lost in this fashion, systems have been suggested for enhancing the agglomeration of the tall oil soap particles prior to their entry into the skimming tank. Since the rate of ascent of these particles is approximately proportional to their size, the agglomeration of the soap particles into larger particles results in a larger portion of these particles reaching the surface of the skimming tanks during the retention of the black liquor in the tanks.
Methods for improving the agglomeration of the tall oil soap particles are disclosed in the patents to Drew, U.S. Pat. No. 3,356,603 and Zucker, U.S. Pat. No. 4,098,673. Each of these patents discloses applying an electrical potential across two electrodes which are immersed within the black liquor so as to encourage the agglomeration of the tall oil soap particles. Zucker, more specifically, discloses disposing the electrodes within the pipe line which carries the black liquor to the skimming tank. The two electrodes are insulated from one another and a D.C. potential applied across them. The polarity of this D.C. potential is such that the upstream electrode is positive and the downstream electrode is negative. According to the Zucker patent, this arrangement produces improved tall oil recovery.